The GB Fuel Log has replaced the Excel spreadsheet I used to use. I only wish it had a MPG calculation that worked from fill-up to fill-up, not requiring 2 fills in a row to calculate, and not leaving any gaps.THANKS GB!

OK, I have thought about it and see where my logic failed. You can always tell exactly how much a partial fill has added to the tank, but you can't tell WHEN that amount has been consumed except by a fill-up or running empty. Sorry for being so dense (I must be coming down with gasheimers).

CK, the idea of a fill up is that you are filling the tank to a consistent, known level.

In your 2nd example if you started with 1/2 tank and ended at 9/16ths tank your calculations would be off. And even if the software allowed you to enter the tank level when you are done adding fuel, it wouldn't ever be particularly accurate - 1/2 vs 9/16ths is hard to read on a fuel gauge but makes an entire gallon of difference in a 16 gallon tank.

Also, fuel gauges are notoriously inconsistent throughout the range. On my Ford Taurus, to go from full to 3/4 tank might take 125 miles, then from 3/4 to 1/2 might take only 100, and from 1/2 to 1/4 might take 75. No way to easily or accurately figure this into the MPG calculation.

I am still puzzled. Once you have had one fill-up to "zero the system" so to speak, if you input all further fuel added (whether partial or fill-up) along with the odometer readings, I would think that would be sufficient to correctly calculate the mileage.

"...the second fill is used to calculated the distance traveled and volume of fuel used from the first." Why wouldn't a partial fill (if the volume and odometer were accurately entered) do just as well? What is so special about a fill-up? As far as I can tell, adding 10 gallons to fill the tank is the same as adding 10 gallons to partially fill it.

After completing your first two fill-ups, the fuel log does calculate from fill to fill.

The two fills per calculation really only applies to the first MPG calculation to get the ball rolling, the second fill is used to calculated the distance traveled and volume of fuel used from the first. After this only one fill is required to the next, because the Odometer reading and the amount of fuel purchased are used to calculate the distance traveled on a given volume of fuel that was used for each previous fill.

You can't have gaps in the calculator, if you do you should use the "missed previous fill - yes" option. This is so the calculator doesn't think you traveled 3000 miles on 15 gallons (example, it's happened) and gives you an unrealistic MPG figure.

If you short fill your tank, you're not getting an accurate calculation.

To use a bad comparison, if you have a near empty 15 gallon fuel tank and the dispenser shuts off at 12 gallons, you'll get a much better fuel economy than if you filled the fuel tank to capacity. Over two (or three, or four) fill-ups you decrease that error and get a much better (long term) reading.

I don't understand why it would be off if the exact amount of fuel purchased at a certain odometer reading is known. The MPG is based on miles divided by gallons; if those two are correct, how can it be off?

An intervening partial purchase, if I understand your question correctly, would mean that the current miles-per-gallon (i.e., between the partial fill-up and the previous fill-up) could be significantly off (the long-term miles-per-gallon would be off, but less so), and would have to be (correctly) offset by an ensured fill-up.

Two fill-ups simply gives a more accurate current miles-per-gallon, which is what most people are interested in, more so than the miles-per-gallon of the past year (in my opinion, though that is NOT saying they're not interested in long-term miles-per-gallon).

If the calculation were to be made from one fill-up to the next, why would an intervening partial purchase(s) (not a fill-up) make any difference as long as the total amount of fuel purchased and mileage between fill-ups is known?

Gas Buddy notes that "To accurately calculate fuel economy (MPG), you must have a minimum of 2 fill-ups in a row where you completely fill the tank."

That's to ensure that your most recent fill-up wasn't inadvertently a short-fill and your calculation is more accurate. A longer (in this case, two fill-up) or larger base to work from makes for better long-term accuracy. Seems to me an accurate calculation would be preferred more than a possibly less accurate one-fill-up gas but that's just my opinion.